In recent years, a technology for adjusting an oscillation frequency by an oscillator having an incorporated resonance circuit in a communication system, such as an optical communication system, has been known. According to this technology, a variable capacitative element, such as a capacitor or a diode, is provided in the resonance circuit to control the capacitance of the variable capacitative element. Thereby, a resonance frequency of the resonance circuit is adjusted, which results in changing a frequency (oscillation frequency) of a clock signal output from the oscillator. By using the oscillator, a clock signal having a high frequency of 20 GHz or more can be output, and a recent requirement for increasing a communication speed or apparatus performance can be met.
When a higher oscillation frequency is needed, an oscillator using a standing wave generated in a transmission path, such as a micro strip line, is used. In this oscillator, the length of the transmission path in the oscillator is determined based on an electrical length of the standing wave. That is, the length of the transmission path in the oscillator becomes ¾ of the electrical length of the oscillation frequency, which is the length where the standing wave is generated. If a voltage is applied to the transmission path, the standing wave is generated on the transmission path. Specifically, a standing wave where a traveling wave propagated to a terminating end of the transmission path and a reflected wave reflected on the terminating end of the transmission path are synthesized is generated. As a result, a clock signal that corresponds to the frequency of the standing wave generated in the transmission path is output from the oscillator. In the case of the oscillator using the standing wave, a variable capacitative element is connected to the terminating end of the transmission path, and the oscillation frequency of the oscillator is adjusted by controlling the capacitance of the variable capacitative element.
Such conventional technologies are disclosed in for example Japanese Laid-open Patent Publication Nos. 2008-118550 and 2005-217752, and Jri Lee et al., “A 75-GHz PLL in 90-nm CMOS Technology”, ISSCC 2007/SESSION 23/BROADBAND RF AND RADAR/23.8, pp. 432-433.
In general, the oscillation frequency of the oscillator has a negative correlative relationship with the capacitance of the variable capacitative element. Specifically, if the capacitance of the variable capacitative element decreases, the oscillation frequency of the oscillator increases, and if the capacitance of the variable capacitative element increases, the oscillation frequency of the oscillator decreases. However, since a variable range of the capacitance of one variable capacitative element is restricted, it is considered to provide plural variable capacitative elements and increase the variable range of the capacitance to flexibly adjust the oscillation frequency of the oscillator over a wide band.
However, when the plural variable capacitative elements are provided, a parasitic capacitance of an entire circuit increases as the number of variable capacitative elements increases. As a result, even though the capacitance of each variable capacitative element is controlled to have a minimum value, the capacitance of the entire circuit is increased by only the parasitic capacitance due to the plural variable capacitive elements. For this reason, a sufficiently high oscillation frequency may not be obtained.
In regards to the oscillator using the standing wave, the oscillation frequency becomes a relatively high frequency, but the variable range of the capacitance of the variable capacitative element needs to be restricted to generate the standing wave which may be used in oscillation. That is, in the oscillator using the standing wave, a phase of the traveling wave at the terminating end of the transmission path is changed by controlling the capacitance of the variable capacitative element. However, if a standing wave where surrounding portions of the terminating end of the transmission path are used as antinodes is not generated, the oscillation frequency cannot be obtained. For this reason, in the oscillator using the standing wave, phases in the surrounding portions of the terminating end of the transmission path cannot be freely adjusted, and the capacitance of the variable capacitative element cannot be greatly changed. In the oscillator using the standing wave, it is difficult to flexibly adjust the oscillation frequency over a wide band, because of a restrictive condition due to the phases.